[llvm] RegAllocGreedy: Fix use after free during last chance recoloring (PR #120697)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-x86_64-expensive-checks-debian` running on `gribozavr4` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/16/builds/11499

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM :: CodeGen/AMDGPU/swdev502267-use-after-free-last-chance-recoloring-alloc-succeeds.mir' FAILED ********************
Exit Code: 2

Command Output (stderr):
--
RUN: at line 2: /b/1/llvm-clang-x86_64-expensive-checks-debian/build/bin/llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx942 -stress-regalloc=4 -start-before=greedy,2 -stop-after=virtregrewriter,2  -o - /b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/test/CodeGen/AMDGPU/swdev502267-use-after-free-last-chance-recoloring-alloc-succeeds.mir | /b/1/llvm-clang-x86_64-expensive-checks-debian/build/bin/FileCheck /b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/test/CodeGen/AMDGPU/swdev502267-use-after-free-last-chance-recoloring-alloc-succeeds.mir
+ /b/1/llvm-clang-x86_64-expensive-checks-debian/build/bin/llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx942 -stress-regalloc=4 -start-before=greedy,2 -stop-after=virtregrewriter,2 -o - /b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/test/CodeGen/AMDGPU/swdev502267-use-after-free-last-chance-recoloring-alloc-succeeds.mir
+ /b/1/llvm-clang-x86_64-expensive-checks-debian/build/bin/FileCheck /b/1/llvm-clang-x86_64-expensive-checks-debian/llvm-project/llvm/test/CodeGen/AMDGPU/swdev502267-use-after-free-last-chance-recoloring-alloc-succeeds.mir

# After Greedy Register Allocator
********** INTERVALS **********


```

</details>

https://github.com/llvm/llvm-project/pull/120697